1. Technical Field
The present disclosure relates in general to methods of drilling wellbores, for example, but not limited to, wellbores for producing hydrocarbons from subterranean formations, and more particularly to methods of using resistivity data ahead of the drill bit to determine pore pressure in the formation as a function of resistivity while the drill bit advances toward but before the bit penetrates a desired region in the formation while drilling.
2. Background Art
Hubbert et al., “Role of Fluid Pressure in Mechanics of Overthrust Faulting, Part I”, Bull., GSA (February 1959) 70, published a comprehensive treatment of the theory of compaction of clay soils by overburden pressure. They showed that as the overburden stress is increased as a result of burial depth, the porosity of a given sedimentary rock is decreased. Therefore, some fluid that was once in the pores of a given formation was later squeezed out by compaction. In many such cases, there is no escape route for the fluid, and thus the fluid becomes overpressured. This happens in many areas, and such generated overpressured zones are often called “abnormal” pressure zones or “geopressure” zones. Geopressure is often referred to as pore pressure.
Formation resistivity measurements are commonly made in oil and gas wells and then used to make decisions about the presence of hydrocarbons, the magnitude of pore pressure, the correlation to formations observed in offset wells, the salinity of formation fluids, porosity of formations, and the presence of permeability. FIG. 1 illustrates graphically the prior art concept of measuring resistivity as a function of depth, showing a typical decrease in resistivity at a depth where increased geopressure (pore pressure) exists (from Eaton, “The Effect of Overburden Stress on Geopressure Prediction From Well Logs”, SPE 3719 (1972)). In shale rocks, resistivity data points diverge from the normal trend toward lower resistivity values, owing to high porosity, overpressured formations.
Existing techniques to measure resistivity are made after the bit penetrates the formation using either electric line logging methods or logging while drilling methods. In either case the formation of interest has already been exposed to the well in order to make the resistivity measurement. This exposure presents problems, including the fact that the condition of the borehole itself and surrounding disturbed formation will have an effect on the very resistivity values being sought, as noted by Hottman et al., “Estimation of Formation Pressures From Log-Derived Shale Properties”, SPE 1110 (1965).
Banning et al. discuss a theoretical application of time-domain electromagnetics (TEM) in a borehole-conveyed logging tool. Banning et al., “Imaging of a subsurface conductivity distribution using a time-domain electromagnetic borehole conveyed logging tool”, Society of Exploration Geophysicists, San Antonio Annual Meeting (2007). See also Published U.S. Patent applications numbers 2005/0092487; 2005/0093546; 2006/003857; 2006/0055411; 2006/0061363; 2006/0061364, and U.S. Pat. No. 6,856,909. Banning et al. state that, theoretically, such a tool may be used to image the conductivity distribution around and ahead of the drill bit at comparatively large distances from the borehole. However, Banning et al. do not disclose or suggest use of resistivity measurements in front of a drilling bit to detect the magnitude of formation pore pressure before the formation is exposed to the well, allowing mud parameters to be adjusted to minimize the risk of a well kick (uncontrolled flow into the well) and/or to reduce the risk of the formation collapsing into the well and sticking the drillpipe (from an overbalanced mud weight condition) and potential loss of the well.
To avoid or reduce undesirable consequences, it would be advantageous if resistivity measurements could be used to make drilling decisions on pore pressure and other formation properties in front of the bit before the bit exposes the formation. In addition, there may be safety and economic advantages gained if a resistivity measurement could be made before the formation was actually exposed to the well. The methods of the present disclosure are directed to these needs.